Polk County is a county located in the Willamette Valley region of the U.S. state of Oregon. As of the 2010 census, the population was 75,403. The county seat is Dallas. The county is named for James Knox Polk, the 11th president of the United States.
The Oregon Provisional Legislature created Polk County from Yamhill District on December 22, 1845, granting to it the entire southwestern portion of present day Oregon to the California border. County boundaries were periodically changed to reflect the creation of Benton and Lincoln counties. Many other counties were subsequently carved out of these as settlement spread towards the south, leaving Polk County many counties away from its former border with California.
The first county seat was a settlement on the north side of Rickreall Creek named Cynthian (also known as Cynthia Ann). In 1852 city officials renamed Cynthian to Dallas after Vice President George M. Dallas, vice president to James Polk. During the 1880s and 1890s, there were a series of unsuccessful efforts to move the county seat to nearby Independence.